Thursday, April 16, 2009

Reading List: Power, Sex, Suicide

When you start to look in detail at the evolution of life on
Earth, it appears to be one mystery after another. Why did
life appear so quickly after the Earth became hospitable to it?
Why did life spend billions of years exclusively in the form
of single-celled bacteria without a nucleus
(bacteria
and
archaea)?
Why are all complex cells
(eukaryotes)
apparently descended from a single ancestral cell?
Why did it take so long for complex multicellular
organisms to evolve? (I've taken a crack [perhaps crackpot] shot
at
that one myself.)
Why did evolution favour sexual reproduction, where two parents are
required to produce offspring, while clonal reproduction is twice as
efficient? Why just two sexes (among the vast majority of species)
and not more? What drove the apparent trend toward greater size and
complexity in multicellular organisms? Why are the life spans of
organisms so accurately predicted by a power law based upon their
metabolic rate? Why and how does metabolic rate fall with the size
of an organism? Why did evolution favour warm-bloodedness (endothermy)
when it increases an organism's requirement for food by more than
an order of magnitude? Why do organisms age, and why is the rate of
ageing and the appearance of degenerative diseases so closely
correlated with metabolic rate? Conversely, why do birds and
bats live so long: a pigeon has about the same mass and metabolic
rate as a rat, yet lives ten times as long?

I was intensely interested in molecular biology and evolution of
complexity in the early 1990s, but midway through that decade I
kind of tuned it out—there was this “Internet”
thing going on which captured my attention…. While much
remains to be discovered, and many of the currently favoured
hypotheses remain speculative, there has been enormous progress
toward resolving these conundra in recent years, and this book is
an excellent way to catch up on this research frontier.

Quite remarkably, a common thread pulling together most of these
questions is one of the most humble and ubiquitous components
of eukaryotic life: the
mitochondria.
Long recognised as the power generators of the cell (“Power”), they have been
subsequently discovered to play a key rôle in
the evolution of sexual reproduction (“Sex”), and in
programmed cell death
(apoptosis—“Suicide”).
Bacteria and archaea are constrained in size by the cube/square law:
they power themselves by respiratory mechanisms embedded in their
cellular membranes, which grow as the square of their diameter, but
consume energy within the bulk of the cell, which grows as the cube.
Consequently, evolution selects for small size, as a larger bacterium
can generate less energy for its internal needs. Further, bacteria compete
for scarce resources purely by replication rate: a bacterium which
divides even a small fraction more rapidly will quickly come to predominate
in the population versus its more slowly reproducing competitors. In cell division,
the most energetically costly and time consuming part is copying the
genome's DNA. As a result, evolution ruthlessly selects for the shortest
genome, which results in the arcane overlapping genes in bacterial DNA
which look like the work of those byte-shaving programmers you knew back when
computers had 8 Kb RAM. All of this conspires to keep bacteria small and
simple and indeed, they appear to be as small and simple today as they
were three billion years and change ago. But that isn't to say they
aren't successful—you may think of them as pond scum, but
if you read the bacterial blogs, they think of us as an ephemeral epiphenomenon.
“It's the age of bacteria, and it always has been.”

Most popular science books deliver one central idea you'll take away from
reading them. This one has a forehead slapper about every twenty pages.
It is not a particularly easy read: nothing in biology is unambiguous,
and you find yourself going down a road and nodding in agreement, only to
find out a few pages later that a subsequent discovery has falsified the
earlier conclusion. While this may be confusing, it gives a sense of how
science is done, and encourages the reader toward scepticism of all
“breakthroughs” reported in the legacy media.

One of the most significant results of recent research into
mitochondrial function is the connection between free radical
production in the respiratory pipeline and ageing. While there is a
power law relationship between metabolic rate and lifespan, there are
outliers (including humans, who live about twice as long as they
“should” based upon their size), and a major discrepancy
for birds which, while obeying the same power law, are offset toward
lifespans from three to ten times as long. Current research offers a
plausible explanation for this: avians require aerobic power
generation much greater than mammals, and consequently have more
mitochondria in their tissues and more respiratory complexes in their
mitochondria. This results in lower free radical production, which
retards the onset of ageing and the degenerative diseases associated
with it. Maybe before long there will be a pill which amplifies the
mitochondrial replication factor in humans and, even if it doesn't
extend our lifespan, retards the onset of the symptoms of ageing and
degenerative diseases until the very end of life (old birds are very
much like young adult birds, so there's an existence proof of this).
I predict that the ethical questions associated with the creation of
this pill will evaporate within about 24 hours of its availability on
the market. Oh, it may have side-effects, such as increasing the
human lifespan to, say, 160 years. Okay, science fiction authors,
over to you!

If you are even remotely interested in these questions, this is a book
you'll want to read.